Method for producing a sheet metal product and sheet metal product

ABSTRACT

A method for producing a sheet metal product (1) from a sheet metal preliminary product (2) is proposed, wherein at least one surface (7) of the sheet metal preliminary product (2) is coated with a coating, wherein the sheet metal preliminary product (2) is rolled, wherein depressions (6) are rolled into the surface (7) during the rolling operation, wherein after the coating and rolling operations lubricant (8.1) is introduced into the depressions (6), wherein after the introduction of lubricant (8.1) the sheet metal preliminary product (2) is mechanically formed by a forming tool (5), characterized in that the surface (7) is coated in such a way that, in the forming tool (5) during the forming process, in particular under relative movement between sheet metal surface and tool surface, the coating is elastically deformable under loads of 0.5 MPa to 20 MPa and plastically deformable under loads of above 20 MPa.

PRIOR ART

The present invention is based on a method for producing a sheet metal product, wherein the sheet metal product is produced from a sheet metal preliminary product by mechanical forming.

Sheet metal preliminary products are usually processed further by means of mechanical forming methods, such as e.g. deep drawing. The forces introduced into the sheet metal material in these methods are transferred to the sheet metal preliminary product via the surface of the sheet. Among other things, the geometry of the sheet metal product and the frictional forces between the surface of the sheet and the forming tool that arise in the forming process generate in some cases very high local loading on the sheet metal surface. This local loadings, which are composed of normal and shear forces, can undergo considerable changes during the forming. For an optimum forming result, it is necessary to be able to selectively control the flow of the sheet during the forming process.

In particular by influencing the retaining forces, which counteract the drawing force of the forming process, the flow of the sheet is controlled and also the blank of the sheet metal preliminary product is adapted. This influencing is brought about by additional lubrication with lubricants, for example. This results in the local reduction of frictional forces and thus in a reduction of the retaining forces. The retaining forces can be increased by the use of drawbeads, for example.

The use of additional lubrication constitutes considerable additional outlay. On the one hand, the lubricants require complex application and, on the other hand, the cleaning outlay for both the sheet metal product and the forming tool increases, because it is possible that the quantities of lubricant introduced can have an adverse effect on the process stability. The avoidable use of lubricants must be viewed critically, in particular against the background of sustainable manufacturing. Document DE 10 2012 017 703 A1 discloses laser texturing methods which make it possible to provide skin pass rollers with a deterministic, i.e. geometrically specific, texture. During the skin passing operation, this texture is transferred as a negative to the surface of the sheet metal preliminary products that are to be processed, i.e. elevations on the roller surface result in valleys in the surface of the sheet metal preliminary product, and vice versa. The depressions introduced into the surface of the sheet metal preliminary product in this way are used as what are known as lubricant pockets, which hold a lubricant applied to the surface of the sheet and can carry it along during the forming.

The functionality of the lubricants used is essentially determined by the additives added to the lubricant. By adding on polymer chains or else by virtue of chemical reactions on the metallic surfaces of forming tool and sheet metal preliminary product, the additives may for example bring about the formation of a boundary layer and prevent direct contact between the surfaces of forming tool and sheet metal preliminary product. In this respect, the bond between the polymer chains is based on van der Waals forces. Therefore, these can be displaced relative to one another comparatively easily. By contrast, the bond between the polymer chains and the surfaces of forming tool and sheet metal preliminary product is based on a dipole bond. The boundary layer is usually a few nanometers thin and can be detached easily from the surfaces of forming tool and sheet metal preliminary product. In order to substantially prevent direct metallic contact and thus wear of the forming tool and the sheet metal preliminary product as a result of adhesion and abrasion, it is therefore necessary to continuously rewet the surfaces with lubricant over the entire surface areas. Reducing the use of lubricant thus contrasts with the requirements of a production process for a sheet metal part according to the prior art.

DISCLOSURE OF THE INVENTION

The invention is therefore based on the technical problem of providing a production method for sheet metal products which does not have the disadvantages of the prior art, but rather offers effective lubrication when the sheet metal preliminary product is being formed combined with reduced use of lubricant.

This object is achieved by a method for producing a sheet metal product from a sheet metal preliminary product, wherein at least one surface of the sheet metal preliminary product is coated with a coating, wherein the sheet metal preliminary product is rolled, wherein depressions are rolled into the surface during the rolling operation, wherein after the coating and rolling operations lubricant is introduced into the depressions, wherein after the introduction of lubricant the sheet metal preliminary product is mechanically formed by a forming tool, characterized in that the surface is coated in such a way that, in the forming tool during the forming process, in particular with a relative movement between the surface of the sheet and the tool surface, the coating is elastically deformable under loads of 0.5 MPa to 20 MPa and plastically deformable under loads of above 20 MPa.

The method according to the invention makes it possible to transport the lubricant in the depressions in the form of lubricant pockets to the critical regions of the forming tool in a targeted manner. The depressions make the surface of the sheet metal preliminary product able to absorb the lubricant and to transport it largely without loss. Only in the event of high loads, i.e. where the lubricant is required for the purpose of lubrication between sheet metal preliminary product and forming tool, is it released locally by a plastic deformation of the depressions.

During the forming operation, such as for example deep drawing, relative movements between sheet metal preliminary product and forming tool occur in particular in the flange or support of the forming tool and at the drawing edge of the forming tool. These regions differ considerably in terms of the loads acting on the sheet metal preliminary product. While in the flange or support there is predominantly areal loading in the single-figure MPa range, at the drawing edge very great loads arise, which can be in particular in the mid-two-figure MPa range and beyond. The depressions are plastically deformed in a manner corresponding to the locally occurring loads, releasing the lubricant. The flow of the sheet during the forming means that further lubricant-filled depressions are continuously conveyed into the regions with very high loads, with the result that these regions are continually rewetted with lubricant.

It is conceivable that the volumes of the depressions correspond to the required quantity of lubricant. For this, it is conceivable that the required quantity of lubricant is between 0.5 g/mm² and 1 g/mm². It is also conceivable that sheet metal blanks or sheet metal in strip form are/is provided as the sheet metal preliminary product. It is furthermore conceivable that, as roller for rolling the depressions into the sheet metal preliminary product, use is made of a skin pass roller, to which a geometric texture has been applied, in particular by a laser texturing process. For the mechanical forming of the sheet metal preliminary product to afford the sheet metal product, all metal sheet forming operations are conceivable. Examples that can be cited here are bending, deep drawing, twisting, pressing and stretching. The coating is preferably between 1 μm and 30 μm thick, in particular between 2 μm and 15 μm thick.

Advantageous configurations and refinements of the invention can be found in the dependent claims, and in the description with reference to the drawings.

According to a preferred embodiment of the present invention, it is provided that the surface is coated in such a way that, in the forming tool during the forming process, in particular with a relative movement between the surface of the sheet and the tool surface, the coating is elastically deformable under loads of 0.5 MPa to 15 MPa and plastically deformable under loads of above 25 MPa. This enables an even better adaptation to conditions prevailing in the forming tool. The existing loads in the forming tool manifest themselves as pressure, i.e. force per unit area, on the surface of the sheet metal preliminary product. A correspondingly advantageous configuration of the depressions makes it possible to manipulate the area on which the force acts. The deformability of the coating can thus be controlled accordingly.

According to a preferred embodiment of the present invention, it is provided that the depressions are rolled in as closed depressions with respect to a main plane of extent of the sheet metal preliminary product. This advantageously allows the lubricant to be transported securely and largely without loss. By virtue of the closed shape with respect to the main plane of extent of the sheet metal preliminary product, no lubricant can run out of the depression laterally. This also ensures that, in the event of plastic deformation of the depression, the lubricant is released and is not pressed away from the region of the plastic deformation along a depression which is not closed.

According to a preferred embodiment of the present invention, it is provided that the depressions are rolled in in the shape of an I, rectangle or square with respect to the main plane of extent of the sheet metal preliminary product. This produces trough-shaped depressions, for example. It has been found that this makes it possible to transport lubricant very effectively. However, it is also conceivable that the depressions are round, oval, triangular or cross-shaped with respect to the main plane of extent of the sheet metal preliminary product.

According to a preferred embodiment of the present invention, it is provided that the depressions are rolled in with a depth of 2 μm to 15 μm, preferably 2 μm to 10 μm, and particularly preferably 2 μm to 5 μm. This enables the volume of the depressions to be adapted well to the required volume of lubricant. Within the meaning of the present invention, the depth of the depression is the depth of the depression after the surface has been coated.

According to a preferred embodiment of the present invention, it is provided that the depressions are rolled in with a ratio of depth to volume of the depression of 1:3 to 1:15, and preferably 1:5 to 1:10. A corresponding ratio furthermore has a positive effect on the transport of lubricant and the selectively local wetting with lubricant. Within the meaning of the present invention, the volume of the depression is the volume of the depression after the surface has been coated.

According to a preferred embodiment of the present invention, it is provided that depressions are rolled in on at least 30% of the area of the surface of the sheet metal preliminary product. This ensures that lubricant is actually also transported to wherever high loads in the forming tool make it necessary to introduce lubricant.

According to a preferred embodiment of the present invention, it is provided that the depressions are rolled in such that a base of the depression has a smaller area than the projection of the depression onto the main plane of extent of the surface. Expressed differently, the depressions are rolled in in the shape of a trough or cup, for example. This is particularly advantageous when the lubricant is being released as the depression deforms.

For this purpose, it is preferably provided that the depressions are rolled in such that walls of the depressions form an angle of 105° to 140° , and preferably 110° to 120° , with the main plane of extent of the surface. The angle between the walls of the depressions and the main plane of extent of the surface makes it possible to selectively set the changes in volume of the depressions owing to the loading. Correspondingly configured angles between the walls of the depressions and the main plane of extent of the surface have proven to be advantageous.

According to a preferred embodiment of the present invention, it is provided that zinc is used as the coating. Zinc is outstandingly suitable as a cathodic corrosion protection coating. When the zinc is processed accordingly, the yield point of the zinc is exceeded at the locations in the forming tool that are exposed to high loading, as a result of which the lubricant is advantageously selectively released.

According to a preferred embodiment of the present invention, it is provided that the sheet metal preliminary product is coated by hot-dip galvanizing, wherein the sheet metal preliminary product is coated before the rolling operation (rolling in of the depressions). Hot-dip galvanizing is well known and is already incorporated to a great degree in production methods. The surface is preferably coated with a closed coating, with the coating not being penetrated when the depressions are rolled in.

According to a preferred embodiment of the present invention, it is provided as an alternative that the sheet metal preliminary product is coated by electrolytic means, wherein the sheet metal preliminary product is coated after the rolling operation (rolling in of depressions). In the case of electrolytic coatings, the coatings follow the uncoated profile of the surface to be coated very precisely. Therefore, the coating may take place after the depressions have been rolled in. This sequence enables a very uniform coating, even within the depressions.

Another subject of the present invention for achieving the object set out in the introduction is a sheet metal product produced by the method according to the invention.

All of the above statements under “Disclosure of the invention” apply equally to the method according to the invention and the sheet metal product according to the invention.

Further details, features and advantages of the invention emerge from the drawings and from the following description of preferred embodiments, with reference to the drawings. In this context, the drawings merely illustrate exemplary embodiments of the invention, which do not restrict the essential inventive concepts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows the method according to an exemplary embodiment of the present invention.

FIG. 2 schematically shows a detail of a sheet metal preliminary product according to an exemplary embodiment of the present invention.

FIG. 3 schematically shows a detail of the surface of a sheet metal preliminary product according to an exemplary embodiment of the present invention.

EMBODIMENTS OF THE INVENTION

In the various figures, the same parts are always provided with the same reference signs and will therefore generally also be specified or mentioned only once in each case.

FIG. 1 schematically illustrates the method according to an exemplary embodiment of the present invention. The sheet metal preliminary product 2 is provided in the form of a strip (coil material) or blank and rolled with skin pass rollers 3. In this context, the skin pass rollers 3 roll trough-shaped depressions (see FIG. 2 ) into the surface of the sheet metal preliminary product 2. The depressions are several micrometers deep, several micrometers long and wide, have a regular arrangement and are rolled in on approx. 40% of the surface. For this purpose, the skin pass rollers 3 have previously been processed, preferably by a laser texturing process, such that they have a corresponding negative on their surface for rolling in the depressions. It should be noted here that the negative has an elevation for each depression, the elevation being higher than the depth of the depression rolled in by the elevation. The reason for this is because, among other things, during the rolling operation the skin pass rollers 3 are not in contact with the surface of the sheet metal preliminary product 2 over the entire surface area, this possibly having negative effects on the rolling/skin passing operation. In order to avoid this, a skin passing liquid is preferably introduced between sheet metal preliminary product 2 and skin pass roller 3, for example.

Following the rolling/skin passing operation, the surfaces are zinc-coated by electrolytic means to a thickness of approx. 7 μm in the coating installation 4. After the coating by electrolytic means, the zinc layer follows the surface geometry of the underlying surface, in particular the rolled-in depressions. As an alternative, it is conceivable that the zinc-coating is performed by means of hot-dip galvanization. In this case, however, the coating would have to be carried out before the rolling/skin passing operation, because a coating applied by hot-dip galvanizing follows the surface geometry of the underlying surface, in particular the rolled-in depressions, less well than coating by electrolytic means. The rolling in of the depressions downstream of the hot-dip galvanization would, however, compensate for this disadvantage again.

In the oiling installation 8, the lubricant 8.1 is now introduced into the depressions on the coated surface. In order to avoid unnecessary costs and to have a positive environmental impact, it should be ensured that essentially only as much lubricant 8 as the depressions can accommodate is introduced.

The rolled and coated sheet metal preliminary product 2 is cut to length from the strip to form a blank and then subjected to forming in a forming tool 5. Deep drawing is illustrated by way of example here as one possible form of the forming operation. The sheet metal preliminary product 2, which is still in the form of a blank, is inserted into the forming tool 5. Outer edges of the sheet metal preliminary product 2 are retained by hold-down devices 5.3 of the forming tool 5 on supports 5.4 of the forming tool 5, while the punch 5.1 presses the inner region of the sheet metal preliminary product 2 into the die 5.2. This produces loads that act on the sheet metal preliminary product 2, including on the hold-down devices 5.3, on the supports 5.4 and on the punch 5.1. These loads are in the single-figure MPa range. Loads likewise act on the sheet metal preliminary product 2 at the drawing edges 5.5. However, the loads here are significantly higher and are situated in a region in which the coating is plastically deformed, the depressions are leveled and the lubricant 8.1 in the depressions is released for the purpose of lubrication. The flow of the sheet from the supports 5.4 in the direction of the drawing edges 5.5 during the deep drawing operation repeatedly transports new depressions to the drawing edges 5.5, which new depressions are likewise leveled there by plastic deformation and release the lubricant 8.1 present therein. This enables continuous and very precisely selective local lubrication. One skilled in the art will understand that the method according to the invention can likewise be used in other forming processes. Bending, twisting, pressing and stretching are mentioned only as further examples, the examples mentioned not being a complete list of the possible forming methods.

FIG. 2 schematically illustrates a detail of a sheet metal preliminary product 2 according to an exemplary embodiment of the present invention. A section orthogonal to the main plane of extent of the surface 7 of the sheet metal preliminary product 2 can be seen. The regularly arranged depressions 6 are several micrometers deep and comprise the base 6.1 and the walls 6.2. The walls 6.2 form an angle α from 110° to 120° with the main plane of extent of the surface 7.

FIG. 3 schematically shows a detail of the surface 7 of a sheet metal preliminary product 2 according to an exemplary embodiment of the present invention. What can be seen are depressions 6, the projections of which onto the main plane of extent of the surface 7 are I-shaped. The surface 7 has the depressions 6 on more than 30% of its area.

LIST OF REFERENCE SIGNS

-   1 Sheet metal product -   2 Sheet metal preliminary product -   3 Roller -   4 Coating installation -   5 Forming tool -   5.1 Punch -   5.2 Die -   5.3 Hold-down device -   5.4 Support -   5.5 Drawing edge -   6 Depression -   6.1 Base -   6.2 Wall -   7 Surface -   8 Oiling installation -   8.1 Lubricant -   α Angle 

1. A method for producing a sheet metal product (1) from a sheet metal preliminary product (2), wherein at least one surface (7) of the sheet metal preliminary product (2) is coated with a coating, wherein the sheet metal preliminary product (2) is rolled, wherein depressions (6) are rolled into the surface (7) during the rolling operation, wherein after the coating and rolling operations lubricant (8.1) is introduced into the depressions (6), wherein after the introduction of lubricant (8.1) the sheet metal preliminary product (2) is mechanically formed by a forming tool (5), characterized in that the surface (7) is coated in such a way that, in the forming tool (5) during the forming process, the coating is elastically deformable under loads of 0.5 MPa to 20 MPa and plastically deformable under loads of above 20 MPa.
 2. The method as claimed in claim 1, wherein the surface (7) is coated in such a way that, in the forming tool (5) during the forming process, the coating is elastically deformable under loads of 0.5 MPa to 15 MPa and plastically deformable under loads of above 25 MPa.
 3. The method as claimed in either of the preceding claims, wherein the depressions (6) are rolled in as closed depressions (6) with respect to a main plane of extent of the sheet metal preliminary product (2).
 4. The method as claimed in claim 3, wherein the depressions (6) are rolled in in the shape of an I, rectangle or square with respect to the main plane of extent of the sheet metal preliminary product (2).
 5. The method as claimed in one of the preceding claims, wherein the depressions (6) are rolled in with a depth of 2 μm to 15 μm, preferably 2 μm to 10 μm, and particularly preferably 2 μm to 5 μm.
 6. The method as claimed in one of claims 3 to 5, wherein the depressions (6) are rolled with a ratio of depth to volume of the depression (6) of 1:3 to 1:15, and preferably 1:5 to 1:10.
 7. The method as claimed in one of the preceding claims, wherein depressions (6) are rolled in on at least 30% of the area of the surface (7) of the sheet metal preliminary product (2).
 8. The method as claimed in one of the claims, wherein the depressions (6) are rolled such that a base (6.1) of the depression (6) has a smaller area than the projection of the depression (6) onto the main plane of extent of the surface (7).
 9. The method as claimed in claim 8, wherein the depressions (6) are rolled in such that walls (6.2) of the depressions (6) form an angle (α) of 105° to 140°, and preferably 110° to 120°, with the main plane of extent of the surface (7).
 10. The method as claimed in one of the preceding claims, wherein zinc is used as the coating.
 11. The method as claimed in one of the preceding claims, wherein the sheet metal preliminary product (2) is coated by hot-dip galvanizing, wherein the sheet metal preliminary product (2) is coated before the rolling operation.
 12. The method as claimed in one of claims 1 to 10, wherein the sheet metal preliminary product (2) is coated by electrolytic means, wherein the sheet metal preliminary product (2) is coated after the rolling operation.
 13. A sheet metal product (1) produced by a method as claimed in one of the preceding claims. 